1. Field of the Invention
The present invention relates to a process for the determination of the correct fuel flow rate to a vehicle engine for carrying out diagnostic tests on a management system for said engine comprising operation sensors.
2. Prior Art
Very complex management systems are increasingly more necessary aboard vehicles, in particular industrial vehicles, for ensuring the correct operation of both the engine under all conditions of use and of the various on-board devices, such as the exhaust gas treatment, the exhaust gas recirculation devices. For example, the fuel injection, the opening of the recirculation line valve, the opening of the variable geometry turbine nozzle, where fitted, are generally controlled by specific control units according to the engine running conditions, the composition of exhaust gases from the engine and the conditions of the treatment devices. The detection of a series of parameters, which may be detected by means of sensors, is thus necessary for the operation of such management systems. Furthermore, the adjustment of the various control units must be sufficiently precise.
The following are among the components more frequently present aboard a vehicle, in particular a vehicle provided with a supercharged engine, and more in particular a diesel engine as those commonly applied to industrial vehicles. An air flow sensor, which is commonly located on the intake line, generally upstream of the supercharging compressor, a supercharging pressure sensor and a supercharging temperature sensor, generally located on the intake line downstream of the supercharging compressor (or compressors if there are more than one, as in the case of multiple stage supercharging or with compressors in parallel) prior to the introduction into the engine, for example the intake manifold. One or various exhaust gas composition sensors: in particular, there is generally a sensor adapted to detect the percentage of oxygen present in the exhaust gases, commonly known as lambda sensor (or probe). The latter is mainly used to adjust the fuel injection, in petrol engines provided with a catalyser. In the case of diesel engines, it is also necessary for a correct adjustment of the engine exhaust gas recirculation flow rate, so as to reduce the generation of pollutants or to guarantee exhaust gas conditions suited to the good operation of treatment systems (catalytic systems, particulate regenerative traps, etc.). Furthermore, in diesel engines there is an exhaust gas recirculation line which appropriately connects the intake line with the engine exhaust line. Various devices (pumps, Venturi tubes) may be provided (in particular in the case of recirculation on the high pressure branch between a point upstream of the turbine on the exhaust line in a point downstream of the intake line compressor, however if a sufficient distance between the recirculation line ends is not otherwise ensured) to allow a suitable flow of recirculated gases under all conditions. Moreover, the adjustment may be performed by means of a valve controlled by an electronic management system. The valve is completely closed if no recirculation is necessary.
The engine is adjusted as shown above according to the values measured by the sensors. The most common problems which may occur include incorrect detection of the intake air flow rate, due to the loss of calibration of the sensor, or to losses on the intake line (with the intake of external air downstream of the sensor if the loss is upstream of the compressor or the loss of air outwards if the loss is downstream of the compressor).
Furthermore, also the temperature and pressure sensors may be subject to error. The lambda sensor may also be subject to malfunctioning or incorrect calibration.
Another common problem is the evaluation error of the recirculated gas flow rate, for example due to valve losses, or other systematic errors, due to incorrect evaluations, for example of the volumetric efficiency (filling) of the engine.
Furthermore, the difficulty in the evaluation of the correct fuel injection flow rate represents a problem. It is indeed known that the flow rate supplied by the injectors is subject to considerable errors (for example approximately 2 mg/cycle) which, at a low load (minor fuel flow rates), may even be 20% of the true value and even exceed 30% when the engine is at a minimum number of rotations, which does not allow to distinguish other possible problems related to the detection sensors of the vehicle.
Since, as mentioned above, among the most common problems there is the incorrect calibration of the air flow rate sensor or errors in evaluating the flow rate due to losses, the control units may not periodically compare the measured flow rate values against a flow rate value calculated as from the supercharging temperature and pressure, the engine speed and the volumetric efficiency (obtainable according to the engine speed from normally available models). The air flow rate sensor may be recalibrated if a significant difference is detected. This method does not account for the fact that there may be other causes of error, whereby leading to the possible generation of systematic errors.
The presence of possible systematic errors is sometimes detected by means of diagnostic tests to be performed in a workshop, for example tests either scheduled or run according to needs. In order to obtain the data detected by the control unit, the control unit may further be connected in a known manner to an external control unit, such as a computer. However, it is often difficult, even if a fault is detected, to trace back to the possible cause without removing the components. Furthermore, the imprecise evaluation of the fuel flow rate represents a considerable limit to the possibility to rapidly identify other problems.
It would be desirable to be able to perform a diagnostic test capable of identifying the component on the basis of possible errors, reducing the need to remove the single components and/or to perform measurements with instruments external to the vehicle.